1. Field of the Invention The present invention relates to a method for making improved lithographic aluminium offset printing plates according to the silver complex diffusion transfer reversal process and to the photosensitive monosheet layer assemblage used for making such printing plates.
2. Description of the Prior Art
The principles of the silver complex diffusion transfer reversal process, briefly called DTR-process herein, have been described in e.g. U.S. Pat. No. 2,352,014.
A lithographic printing plate can be made according to the DTR-process. In U.S. Pat. No. 3,511,656 a method has been described for making a printing plate by photo-exposing a material comprising in the given sequence a silver halide emulsion layer, a silver-receptive stratum containing nuclei for precipitation of silver from diffusing water-soluble silver complexes, and a base sheet e.g. an aluminium foil, and applying an aqueous alkaline solution of a developing agent and silver halide solvent to the photo-exposed silver halide emulsion layer, reducing the exposed silver halide, allowing the unreduced silver halide or complexes formed thereof to diffuse from the unexposed areas of the silver halide emulsion layer to the silver-receptive stratum to produce from the unreduced silver halide or complexes formed thereof in conjunction with the nuclei a visible silver image in the silver-receptive stratum, said image being oleophilic ink-receptive, and removing the photo-exposed silver halide emulsion layer from the surface of the silver-receptive stratum with warm water. Printing can be achieved by wetting the imaged silver-receptive stratum with aqueous dampening liquid to wet out the non-imaged areas, coating the silver-receptive stratum with an ink, which wets out the imaged areas, and pressing the inked surface onto copy sheets for the transfer of the ink image thereto. It is possible also to dispense with the silver-receptive stratum containing nuclei so that the oleophilic ink-receptive image is formed directly on the base sheet e.g. an aluminium foil, the surface of which has been rendered hydrophilic previously by brushing, silicating, anodizing, etching, or the like. By treatment with a lacquer the oleophilicity of the silver image can be increased, if desired.
In U.S. Pat. No. 4,772,535 a light-sensitive lithographic printing plate material has been described, which material comprises a support e.g. a metal support, an optional subbing or antihalation layer or undercoat, a non-light-sensitive silver halide emulsion layer, a light-sensitive silver halide emulsion layer, and an image-receiving layer containing physical development nuclei. The material is exposed image-wise through the image-receiving layer and developed to form a diffusion transfer silver image in the image-receiving layer (not in the metal support). The imaged element thus obtained is used as such as a printing plate without separation of the now useless emulsion layers from the layer that contains the printing image.
According to EP-A 0,278,766 a lithographic printing plate precursor has been proposed, said precursor comprising a grained and anodized aluminium foil coated with a sol containing nuclei in a gelatin binder and--according to one embodiment--covered with a silver halide emulsion layer. Extensive experimentation with a said printing plate precursor has shown unfortunately that satisfactory printing results can only be obtained on the condition that after development of said precursor, the residual emulsion layer is removed by washing with hot water (50.degree. C.) and that the image plate is treated with a finisher comprising large amounts (20 g/l) of trypsin. The use of hot water has several disadvantages. The cost of hot water is high. Moreover, hot water dissolves the proteinic binder, usually gelatin, of the emulsion layer, thus causing decomposition of said layer so that a dirty black waste water comprising silver particles and dissolved silver salts is obtained, which upon cooling may clog filters and draining pipes. As for trypsin, this is a proteolytic enzyme that should be present in the finisher to degrade or decompose the proteinic binder that has adsorbed onto the silver grains precipitating on the aluminium foil during image formation. Substantial amounts of proteinic binder can indeed easily reach the silver grains owing to the fact that a silver-receptive stratum comprising gelatin and a gelatin silver halide emulsion layer have been coated directly on the aluminium foil. After oleophilization of said silver image the adsorbed gelatin, which is inherently hydrophilic, constitutes an undesired hydrophilic element in the master image so that prints having an insatisfactory quality are obtained. Moreover, said trypsin, which is essential to degrade the proteinic binder in the silver image, is extraordinarily expensive and is ecologically harmful as can be derived from i.a. Sigma Aldrich Library of Chemical Safety Data: MSD Book, 2,35553A,B,C and from Registry of Toxic Effects of Chemical Substances, YN507500.
In addition to the above disadvantages it has also been established that the gelatin present in substantial amounts in the nuclei-containing layer and in the emulsion of the lithographic printing plate precursor layer has a corrosive effect on the aluminium foil. The corrosive effect of gelatin on aluminium has indeed been described by J. H. Penn and G. A. W. Murray in Br. Corros. J., 1967, Vol .2, September, pages 193-4. Even though the corrosive influence of gelatin on the aluminium foil may be limited thanks to the presence of the anodization layer thereon, this protection is incomplete owing to random defects in the continuity of the anodization layer.
Furthermore, it is generally known that aluminium ions have a hardening influence on gelatin (see e.g. the paragraph bridging pages 78 and 79 of "The Theory of the Photographic Process" 4th Ed., edited by T. H. James). Aluminium ions of the foil can indeed cause a hardening reaction in the gelatin layers so that removal of the emulsion layer gradually becomes more difficult.
Finally, as a result of the corrosive effect of gelatin on aluminium and the hardening reaction caused by aluminium in the gelatin layers, the shelf-life of the lithographic printing plate precursor is limited substantially.
According to U.S. Ser. No. 07/552,945 these disadvantages have been circumvented for the major part by providing between the aluminium foil and the silver halide emulsion layer a thin water-swellable intermediate layer comprising for at least 70% of its total weight at least one non-proteinic hydrophilic film-forming polymer.
However, since on a microscopic scale the grained surface of an aluminium foil is very rugged with deep valleys and steep peaks, it is practically impossible to completely cover this rugged aluminium surface with just a thin water-swellable intermediate layer. It was, therefore, tried to fill up this ruggedness by enhancing the thickness of the water-swellable intermediate layer, but although a thicker intermediate layer offered a solution with respect to creating a more efficient barrier between the aluminium surface and the emulsion layer, it led to another disadvantage. During diffusion transfer the silver salts migrating from the emulsion layer to the aluminium surface through the thick water-swollen intermediate layer have to cover a longer diffusion path so that lateral diffusion becomes more substantial. As a consequence, the deposition of silver on the aluminium surface and the sharpness of the transferred silver image are reduced.